SCIENTISTS AGREE THAT the climate is changing, but they debate the extent of the danger. In the face of that disagreement, many people are asking: If we are uncertain, shouldn't we take aggressive action? The planet is at risk, the argument goes, and so it would be prudent to take bold steps immediately.
Similarly, experts debate the threat posed by low levels of arsenic in drinking water, by genetically modified food, by particulates in the air. While the experts argue, many citizens are asking: Isn't it better to be safe than sorry?
These apparently sensible questions have culminated in an influential doctrine, known as the precautionary principle. The central idea is simple: Avoid steps that will create a risk of harm. Until safety is established, be cautious; do not require unambiguous evidence. The principle, in its many variations, has come to play a powerful role in public debate, the development of government policy, and even international law. It can be, and has been, applied to countless problems, including nuclear power, cellphones, pesticides, electromagnetic fields, and even human cloning.
Yet the precautionary principle, for all its rhetorical appeal, is deeply incoherent. It is of course true that we should take precautions against some speculative dangers. But there are always risks on both sides of a decision; inaction can bring danger, but so can action. Precautions, in other words, themselves create risks - and hence the principle bans what it simultaneously requires.
Consider the Iraq war. At times, the Bush administration justified the war on explicitly precautionary grounds - that even the possibility of a nuclear-armed Iraq was so threatening that it demanded action. Indeed, the idea of "preemptive war" articulated by President Bush is a kind of precautionary principle. The nation went to war on the chance that Saddam Hussein had weapons of mass destruction. But this precaution is imposing a heavy price and creating serious risks for the future.
War is unique, but the same point holds in other contexts, including the domain of climate change, in which costly precautions inevitably create risks. This is not to say that we should not take action to avert the dangers posed by climate change; we should. But if we take steps to reduce risks, we will always create fresh hazards. No choice is risk-free. For environmental and other problems, we need to decide which risks to combat - not comfort ourselves with the pretense that there is such a thing as a "safe" choice.
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The precautionary principle was initially developed in Germany and Sweden in the late 1960s. The first legal use of the principle appears in the Swedish Environmental Protection Act of 1969. In the same period, German environmental policy was founded on the basis of Vorsorgeprinzip, a precursor of the precautionary principle. In the last 15 years, the principle has been used in many international agreements.
The most limited versions of the principle suggest, quite sensibly, that a lack of decisive evidence of harm should not be grounds for refusing to respond. Controls might be justified even if we cannot establish a definite connection between, for example, low-level exposures to humanly-introduced carcinogens and adverse effects on human health. Thus the 1992 Rio Declaration, setting out principles for sustainable development, states, "Where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation."
In some places, however, the precautionary principle is understood in a stronger way, asking for a significant margin of safety for all decisions. For example, the Final Declaration of the First European "Seas At Risk" conference in 1994 says that if "the 'worst case scenario' for a certain activity is serious enough then even a small amount of doubt as to the safety of that activity is sufficient to stop it taking place." Likewise, some people argue that because genetic modification of food creates ecological risks, because the pesticide DDT threatens human health (and wildlife), and because nuclear power has a really bad worst-case scenario, we should impose immediate bans.
The simplest problem with the precautionary principle is that regulation might well deprive society of significant benefits, and even produce a large number of deaths that would otherwise not occur. In some cases, government regulation eliminates the "opportunity benefits" of a process or activity, and thus threatens to cause preventable deaths.
Consider the case of genetically modified food. Many people fear that "tampering with nature" could produce adverse consequences for our health and for the environment. But others argue that a failure to allow genetic modification might well result in numerous deaths, and a small probability of many more. The reason: Genetic modification holds out the promise of producing food that is both cheaper and healthier - resulting, for example, in products that might have large benefits in developing countries. The point is not that genetic modification will definitely have those benefits, or that the benefits of genetic modification outweigh the risks. The point is that the precautionary principle provides no guidance.
Regulations sometimes give rise to substitute risks. DDT, for example, is often regulated in the interest of protecting birds and human health. In poor nations, though, DDT bans eliminate what appears to be the most effective way of combating malaria - and thus significantly undermine public health.
Or consider the "drug lag," produced whenever the government takes a highly precautionary approach to the introduction of new drugs. Stringent review protects people against inadequately tested drugs; but it will also prevent people from receiving the benefits of new medications. Is it "precautionary" to require extensive testing, or to do the opposite?
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In light of its incoherence, why does the precautionary principle seem so appealing? Part of the answer is psychological. Particularly powerful is the belief that nature is benevolent and harmonious. Studies show that people overestimate the carcinogenic risk from pesticides and underestimate the risks of natural carcinogens. People also believe that nature implies safety, so much that they will prefer natural water to processed water even if the two are chemically identical. Most people believe that natural chemicals are safer than man-made chemicals. Most toxicologists disagree.
The truth is that nature is often a realm of destruction, illness, killing, and death. Human activity is not necessarily or systematically more destructive than what nature does.
Nothing is more natural than exposure to sunlight, which people rarely fear. But this exposure is associated with skin cancer and other serious health problems. Tobacco smoking kills 400,000 Americans each year, even though tobacco is a product of nature. To say all this is not to resolve specific issues, which depend on complex questions of value and fact. But the false belief in a benevolence of nature helps to explain the appeal of the precautionary principle.
Another reason for the appeal is what psychologists call loss aversion: In people's minds, a loss from the status quo is more bad than an equal gain is good. When we anticipate a loss of what we now have, we can become genuinely afraid. People will be closely attuned to the losses produced by a newly introduced risk, but far less concerned with benefits forgone.
Loss aversion is closely associated with another cognitive finding: People are far more willing to tolerate familiar risks than unfamiliar ones, even if they are statistically equivalent. For example, the risks associated with driving do not occasion a great deal of concern, even though in the United States alone, tens of thousands of people die from motor vehicle accidents each year. Driving is simply seen as a part of life. By contrast, many people are quite concerned about risks that appear newer, such as the risks associated with genetically modified foods, recently introduced chemicals, and terrorism.
If you were to try to follow the precautionary principle in the next 24 hours, you'd face a real challenge. Not exercising imposes risks; exercising imposes risks. Hard work can be hazardous, but if you don't work hard, you might irritate your employer. And if you do that, you'll be imposing some risks on yourself and your family.
In the context of climate change, precautions are certainly a good idea. But what kinds of precautions? A high tax on carbon emissions would impose real risks - including increased hardship for people who can least afford it and very possibly increases in unemployment and hence poverty. A sensible climate change policy balances the costs and benefits of emissions reductions. If the policy includes costly (and hence risk-creating) precautions, it is because those precautions are justified by their benefits.
The nations of the world should take precautions, certainly. But they should not adopt the precautionary principle.
Cass R. Sunstein is Felix Frankfurter professor of law, Harvard Law School, and coauthor, most recently, of "Nudge: Improving Decisions About Health, Wealth, and Happiness." This was adapted from an article in the spring issue of Daedalus.